1. Field of the Invention
The present invention relates to a variable-displacement turbine, and more particularly to a variable-displacement turbine for use in a turbocharger for an internal combustion engine.
2. Description of the Relevant Art
As disclosed in Japanese Laid-Open Patent Publication No. 62-282122, the variable-displacement turbine of a turbocharger for an automotive internal combustion engine includes variable nozzles disposed in an exhaust passage for guiding exhaust gases to a turbine wheel. The opening of the variable nozzles is adjusted according to operating conditions or the like of the internal combustion engine to control the speed of flow of the exhaust gases which impinge upon the turbine wheel. More specifically, the variable-displacement turbine includes a turbine housing defining a scroll passage which opens toward the outer periphery of the turbine wheel for guiding the exhaust gases to swirl and impinge upon the turbine wheel. The scroll passage houses therein an array of alternate stationary and movable vanes extending radially outwardly of the turbine wheel, the stationary and movable vanes jointly defining the variable nozzles between their confronting surfaces.
The opening of the variable nozzles can be adjusted by tilting the movable vanes. A wall of the scroll passage has engaging portions such a steps for engaging the movable vanes to limit the angular displacement of the movable vanes thereby establishing a minimum opening of the variable nozzles. The steps are shaped in complementary relation to the configuration of the movable vanes.
The variable-displacement turbine disclosed in the above publication increases the controllability of the turbocharger since it provides a larger range of variable openings of the variable nozzles.
However, the disclosed variable-displacement turbine must be machined with high precision because the vane-shaped steps for establishing the minimum opening for the variable nozzles need to be machined highly precisely. As a result, the process of manufacturing the variable-displacement turbine is complex and highly costly. The publication referred to above also shows an embodiment which has two stationary vanes and two movable vanes. With such fewer movable vanes, however, nozzles or ports for supplying exhaust gases from the scroll to the turbine wheel are largely spaced from each other, a configuration which may adversely affect the efficiency of the turbine.